1. Field of the Invention
The present invention relates generally to an apparatus which employs a high frequency electromagnetic wave energy, and more particularly to a multimode electromagnetic wave energy rejection filter arrangement for preventing leakage of electromagnetic energy from such apparatus.
2. Background of the Invention
Various techniques utilize high frequency bands in the communication field. It is well known to those skilled in the art that the demand for electromagnetic compatibility of equipment and, hence, low levels of apparatus interference is extremely important due to the active electromagnetic environment of communication systems. For that reason the problem of handling electromagnetic energy leaks has attracted considerable attention in the past years.
Nowadays, this problem has acquired a particular interest in terms of personal safety, once microwave devices have found extensive application in areas beyond scientific research and communication systems, such as food processing industry, medicine, household appliances, devices for heating of dielectric materials as, for example, domestic microwave ovens. The most advanced solutions to the problem of preventing electromagnetic leaks have been offered, naturally, in the latter application.
The arrangement for prevention of leakage of high frequency electromagnetic waves will be described herein by taking, as an example, a domestic microwave oven which utilizes high frequency electromagnetic energy for cooking or heating food items.
Generally, a microwave oven includes a magnetron for generating microwave and a resonator in which the microwave energy is employed for heating, the resonator having a body with an access opening and a door member installed so as to allow opening and closing of the access opening, the door member periphery enveloping the access opening. In the available microwave oven designs there is always a gap between the body of the resonator and the door member periphery when the access opening is closed. This gap acts a slot waveguide through which the electromagnetic energy leaks from the resonator.
Traditionally the problem of preventing this leakage is solved by using absorbing materials or installing a rejection filter either along the perimeter of the door member or along the perimeter of the access opening.
The rejection filter is commonly designed as a quarterwave distributed-parameters choke placed along the perimeter of the door member so that it is coupled with the slot waveguide cavity via a coupling hole. An example of a rejection filter of this type is disclosed in a U.S. Pat. No. 4,584,447, wherein the filter proves quite effective in preventing leakage of high-frequency energy from the microwave oven resonator at the fundamental frequency, but it is insensitive to leakage at higher harmonics. Besides, this filter can provide effective rejection of waves only within a limited range of their angles of incidence.
All efforts towards upgrading the rejection filter design so far have pursued better characteristics and compactness of the device at a lower cost. One example is a multimode rejection filter for a microwave oven described in the European Pat. No. 0,196,214. The filter contains two quarter-wave distributed-parameters chokes, one bing tuned to the fundamental frequency of the cavity, the other being tuned to the frequency of a higher (for example, 5th) harmonic.
However, this design fails to provide an appreciably wider range of angles of incidence of the waves, wherein the filter will be effective, due to a strong coupling between the elements of the choke (i.e. a high value of mutual inductance).
The multimode rejection filter for a microwave oven according to the U.S. Pat. No. 5,075,525, comprises two quarter-wave distributed-parameters chokes, one of which being tuned to the fundamental frequency of the resonator, the other being tuned to a higher harmonic frequency, and two capacitance filters, The advantage of this arrangement is a slightly expanded range of propagation directions of the waves that are being rejected. Its major shortcoming, though, is the design complexity, a rather large size of the choke, and a nonuniform dependence of the reflectivity of waves to be rejected on their propagation directions.
The microwave oven according to the U.S. Pat. No. 4,700,034, incorporates a multimode rejection filter which is coupled with the slot waveguide cavity through a coupling hole and is essentially a series LC-circuit consisting of distributed-parameters elements. The circuit is placed inside a quarter-wave choke along a line enveloping the access opening. This filter provides good rejection of waves at the fundamental--and the 2nd--harmonic frequencies but only within a limited range of directions of propagating waves, which is the major drawback in this design. Besides, it is quite complicated structurally, which makes the overall design of microwave oven more sophisticated.
The closest analog of the present multimode rejection filter for a slot waveguide in terms of collective relevant properties featured in both designs is the multimode rejection filter proposed in the paper: "High response door seal for microwave oven" by S. Ohkava, H. Watanabe, K. Kane (Microwave Power Symposium Digest, 1978). This filter is intended for utilization in domestic microwave ovens to prevent leaks of high-frequency energy from a resonator heating chamber.
The filter comprises a system of strongly coupled series LC-circuits arranged inside a quarter-wave choke along a predetermined line intersecting the wave vectors of the waves to be rejected. It also comprises a parallel LC-circuit located inside another quarter-wave choke. Both of these quarter-wave chokes are coupled with a slot waveguide cavity via a coupling hole, the slot waveguide being formed by a body of the resonator heating chamber and a door member periphery enveloping an access opening in the heating chamber. The LC-circuits consist of distributed elements.
The disadvantages of this multimode rejection filter are the design complexity and the dependence, although a weaker one as against other filter designs used for the same purpose, of the transmittance on the propagation direction of waves to be rejected. This limits the range of angles of incidence of the waves rejected by the filter.